Print Rotation Matrix From Variable?
Your script is fine, i just tested and it work as expected. The problem maybe is that you are looking in the python interactive console, but you need to look in the system console.

python interactive console after run script:



System console after run script:



If you are in GNU/Linux os look in the terminal or run blender from a terminal.

In Windows got to:  Window>Toggle System console:



mat_rot = eul.to_matrix() #Convert rotations to matrix

mat = mat_rot.to_4x4() #Lets turn this into a grid!

print(mat)
print(mat[0])
print(mat[0][0])